1. Field of the Invention
This invention relates to rubber compositions suitable particularly for use in pneumatic tire treads.
2. Prior Art
Automotive tires should be safe to drive, inexpensive to manufacture and comfortable to ride on. As a result of the development of networks of expressways, there has been an urgent need for improved means capable of giving reliable steering, cornering and braking characteristics while in high-speed driving.
To attain improved tire performance particularly in terms of friction performance of tires with the surfaces of roads or pavements, it is important that tread rubber be made highly susceptible to hysteresis loss thereby leading to sufficient friction force between the tread and the road surfaces. Hysteresis loss results from periodic deformation of the tread on its surface while in high-speed, frictional contact with slight undulations on the roads. The more frequently the tread dissipates energy at the contacting areas through hysteresis loss, the greater the friction force. In accordance with the Williams-Landel-Fery temperature-time superposition principle, such tread deformation is known to be dependent upon the magnitude of hysteresis loss determined at a lower temperature than that at which the tire is used. In fact, loss factor (tan .delta.) that is a measure of the hysteresis loss is fairly correlative, when measured particularly at about 0.degree.C., to the friction coefficient that the tire has gained.
Attempts have been made to this end to enhance hysteresis loss with use of styrene-butadiene copolymer rubbers having high styrene contents and high glass-transition temperatures. Conventional rubbers of a styrene-butadiene type, however, tend to invite insufficient abrasion resistance with increased styrene contents and also inadequate low-temperature resistance at elevated glass-transition temperature. These rubbers when exposed to reduced ambient temperature would give too high elastic moduli to follow the road surface irregularities; that is, energy dissipation would be rather small despite the tan .delta. values held high, resulting in reduced braking force. At much lower ambient temperature the rubber would undergo brittle fracture, causing serious accidents.